The present invention relates to a method for reading out and writing to RFID-transponders with an inductive coupling, using a read/write unit, wherein the transponders operate at a resonance frequency. The invention further relates to a system for reading out and writing to RFID-transponders, wherein the system comprises a transponder, which is disposed on a carrier and has a resonant circuit with a resonance frequency, a receiving unit and a read/write unit with a transmitter/receiver.
RFID-systems are becoming increasingly more frequently used for means of contact-free, automatic identification purposes. Approx. 90% of all sold RFID-systems are nowadays inductively-coupled systems with an inductive coupling between the reading device and the transponder. Such systems, so-called remote-coupling systems generally function in the ranges of up to 1m in the read/write operation.
The transmission frequencies used are frequencies below 135 kHz or the frequencies 6.78 MHz, 13.56 MHz and 27.125 MHz, i.e. the ISM frequency ranges which are held free especially for industrial, scientific or medical purposes. Depending upon the frequencies used, differences occur in the data transmission rates, clock frequencies, output, etc.
It is possible nowadays to use RFID-labels to identify goods and other objects as these labels can now be produced practically as thin as conventional adhesive labels and thus are not generally recognised by the user as being RFID-labels. For example, RFID-labels can be adhered or laminated to books, periodicals or similar documents.
RFID-systems which function in the radio frequency range (3 MHz to 30 MHz) operate with LC-resonant circuits at a resonance frequency fR. If a magnetic alternating field acts with a frequency fs on the transponder of an RFID-label, then the resonant circuit of the transponder starts to respond and is excited to resonance oscillation. In so doing it takes energy from the magnetic alternating field, which for example can be acquired by increasing the coil current or the voltage drop at the internal resistor in the transmitter circuit. In this manner the operating voltage for the transponder chip can also be produced.
In the case of EAS-systems, i.e. electronic article security, a warble frequency is used. The transmitter frequency continuously scans a frequency range. This can be recognised by the energy absorption which occurs at an unknown resonance frequency of a transponder. DE 195 14 601 A1 describes an EAS-system of this type which has a broadband pre-amplifier which for example in the case of two transponder types passes through their two frequency ranges one after the other.
The drop in voltage in the transmitter circuit as a consequence of the receiving transponder being excited to oscillate is exploited during load modulation, when by means of switching on and off the load resistor of the transponder voltage changes are caused at the antenna of the transmitter and thus amplitude modulation of the antenna voltage is effected.
If two RFID-labels are located in close proximity to each other, possibly stacked one on top of the other in a document file or adjacent to each other on a bookshelf, then these have a mutual effect on each other during reception, i.e. they receive approximately with identical strength an in-phase signal from the transmitter and coupling effects occur. If they lie precisely one above the other, then there is practically a common coil, wherein the two capacitors are connected in parallel. Thus a frequency displacement occurs, i.e. a change in the resonance frequency. This leads to the relevant read device only being able to receive the data in a limited form or not being able to receive the data at all.
Tests have shown that the resonance frequency of an RFID-label always displaces downwards when a second RFID-label comes into the coupling range of the first. In the extreme case, a rigid coupling can occur, wherein the resonance frequency of the two RFID-labels then amounts to
fR/{square root over (2)}
Even when using high field strengths, i.e. high transmitter output, it is not always possible to communicate at the transmission frequency fs. Depending upon the coupling level and resonant frequency of the transponder circuits, zero settings can lie above the resonance frequency which has been displaced downwards. If such a zero setting occurs particularly where the transmission frequency fs is used, then the chip no longer functions.
This is illustrated with reference to the replacement circuit diagrams in FIG. 4. There is located in the transmitter branch of the transmitter/receiver A an oscillator 2 with a frequency fs, whose output signal is transmitted, where appropriate after modulation, to an output end phase 4. The receiving branch which commences immediately at the antenna bush is provided with a demodulator 6 and a band pass filter 8 or a different filter. The antenna is a coil 10 with inductivity Ls. The replacement circuit diagrams of two RFID-labels a,b are also illustrated. These include in each case a coil 12 with inductivity L1 and L2 and a capacitance 14 parallel to the transponder chip 16. The coupling ratios are illustrated by lines, wherein ks represents the coupling transmitter/receiver and the transponder and k represents the coupling between the two transponders. If there are differences in size (and thus coupling) or frequency of the RFID-labels, then the mentioned deletions occur.
FIGS. 5 and 6 illustrate simulations of this process of the undesired coupling between two adjacent RFID-labels with like and unlike resonance frequency.
In the first example (FIG. 5) the frequency responses for transponders with identical resonance frequency are illustrated, where there is no coupling (k=0) at 13.56 MHz and total coupling (k=1) at approx. 10 MHz. Thus, as the coupling increases the occurring resonance frequency drops to lower frequencies.
In the case of different resonance frequencies as shown by the second example illustrated in FIG. 6, there is in fact again no coupling (k=0) at 13.56 MHz and the coupling increases [sic] to lower frequencies, wherein again total coupling (k=1) occurs at approx. 10 MHz. However, a zero setting x has formed just above 13.56 MHz, so that under certain conditions the chip can fail.
It is known to use RFID-systems which operate in the microwave range to eliminate randomly occurring interference signals, where the transponders operate at several resonance frequencies. This is the case with an RFID-system described in U.S. Pat. No. 5,446,447 for reducing the read time.
Furthermore, RFID-systems with transmission frequencies in the microwave range have been used, where the transmission signal is modulated with a signal of e.g. 1 kHz and in addition to the resonance frequency the second harmonic of the transponder is also detected. Following demodulation and the passing through of a 1 kHz detector it is possible in a reliable manner to distinguish between the received transponder signals and interference signals and thus false alarms are avoided. However, disadvantages of these RFID-systems are the influences exerted by the multi-path and running-time effects.
The object of the invention is to provide a method which renders it possible to perform a reliable operation, in particular communication and antenna feeding, even if several transponders are located over a large spatial area.
Thus, in the case of the method in accordance with the invention for reading out and writing to RFID-transponders with inductive coupling using a read/write unit, the transponders operate at a fixed resonance frequency. The transmission frequency is reduced from a transmission basic frequency according to this resonance frequency for operating conditions at a high range to a fixed alternative value of the transmission frequency for operating conditions with a high recognition rate, so that a reliable communication between transponder and read/write unit is guaranteed.
The method in accordance with the invention is suitable for use in the range of approx. 10 kHz to approx. 30 MHz. It is preferably used in the radio frequency range. In preference, the following values are provided as frequency combinations:
a) fR=13.56 MHz; fs1=13.56 MHz, fs2=6.78 MHz
b) fR=27,125 MHz; fs1=27,125 MHz, fs2=13.56 MHz, fs3=6.78 MHz
The manufacturing tolerances allow the transponder resonance frequency to be varied by approx.xc2x12%.
By providing two operating frequencies it is possible on the one hand to eliminate interference caused by other transponders located in the proximity or in any case to reduce it greatly and on the other hand to operate with the necessary range. In an advantageous manner the resonant circuits of the transponders are adjusted so that they individually operate to an optimum on the transmission basic frequency and also have the greatest range there.
If it is to be assumed that in the proximity of a transponder there are other transponders whose signals could interfere with the communication, the operating frequency, i.e. the transmission frequency of the transmit/read unit, is reduced from about 13.56 MHz to the (or a) lower value 6.78 MHz which thus lies further away from the resonance frequency 13.56 MHz of the transponder. As a consequence, the likelihood of interference is reduced and the recognition rate is increased, as is necessary for densely packed RFID labels. It is thus possible in the case of differing inquiry frequency and resonance frequency to detect with a reliable recognition rate many transponders with the same resonance frequency simultaneously in the read field. The transmission range is automatically lower in the case of a reduced transmission frequency. If, on the other hand, a higher range (recognition distance) of the transmitter is required and less interference is anticipated, then the higher transmission frequency is set. The method in accordance with the invention thus provides for the same transponders to operate with different transmission devices irrespective of the on-site conditions.
Thus, for example, a gate system at the exit of a library, a warehouse or a sales outlet can transmit at the basic frequency (e.g. 13.56 MHz), since little interference from other transponders is to be expected at this location. On the contrary, the transmission range here is greater. However, the read/write units of the warehouse and sales management operate at a lower alternative transmission frequency (e.g. 6.78 MHz) as here the range is not so important, but extensive protection against interference is.
The system in accordance with the invention for reading out and writing to RFID-transponders which is suitable in particular for implementing the method in accordance with the invention comprises a transponder which is disposed on a carrier and has a resonant circuit with a resonance frequency, a receiving unit and a read/write unit with a transmitter/receiver, wherein the resonant circuit has a fixed resonance frequency and the transmitter/receiver of the read/write unit has a lower transmission frequency (transmission frequencies) according to the resonance frequencies of the transponder as an alternative to the basic transmission frequency.
In so doing, transmitters having several adjustable transmission frequencies, for example 13.56 MHz and 6.78 MHz can be used in many ways.
The system in accordance with the invention can preferably be used with extremely thin labels which are embedded in the most varied of goods.
The method and system in accordance with the invention for reading out and writing to RFID-transponders can be used as a replacement for optical barcode systems (retail, logistics, warehouse management); as a replacement for SmartCards (payment cards, guarantee cards, discount cards); as means of identification (books, documents, passes, tickets, certificates); to ensure copyright protection (books, dresses, sound media). This list is merely an example and is by no means complete. In comparison to the conventional systems, when using RFID-transponders the significant advantages are a high read rate and the lack of dependency on the positioning, meteorological conditions and the absence of wear. One or more transponders can be supplied with sufficient operating voltage, so that these can become active and can produce a response by means of load modulation.
This response is transmitted synchronously with respect to the carrier signal but on an integral ratio thereto, wherein the clock in the transponder chip is obtained by separation from the transmission signal. In one embodiment an amplitude-modulated auxiliary carrier with a frequency fs/32 is used as the return channel, i.e. the return channel is on fs=fsxc2x1fs/32.